Prolymphocytic leukaemia occurred in two women aged 75 and 57 years, respectively. In both instances the lymphatic cells fulfilled the morphological criteria of the disease picture. In one patient the disease was characterised by immunological and physical methods as being a B-cell lymphoma, in the other a T-cell lymphoma. Acid phosphatase was of special significance among cytochemical studies. While the patient with the T-cell lymphoma died after three months, the one with B-cell lymphoma is still alive 16 months later. Splenectomy resulted in marked improvement in the latter patient's condition and may turn out to be the treatment of choice in prolymphocytic leukaemia.